Organic light emitting devices, which make use of thin film materials which emit light when excited by electric current, are becoming an increasingly popular technology for applications such as flat panel displays. A typical such organic emissive structure is referred to as a double heterostructure (DH) OLED, shown in FIG. 1A. In this device, a substrate layer of glass 10 is coated by a thin layer of indium-tin-oxide (ITO) 11. Next, a thin (100-1000 .ANG.) organic hole transporting layer (HTL) 12 is deposited on the ITO layer 11. Deposited on the surface of HTL 12 is a thin (typically, 50 .ANG.-1000 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-1000 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
Often, the EL 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and a thick, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17" is opaque. When proper bias voltage is applied between a top electrode 17 and contacts 15 and 16, light emission occurs from emissive layer 13 through the glass substrate 10. An OLED such as that of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05% to 2% depending on the color of emission and the device structure.
Another known organic emissive structure referred to as a single heterostructure (SH) is shown in FIG. 1B. The difference between this structure and the DH structure is that multifunctional layer 13' serves as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13' must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
Yet another known OLED device is shown in FIG. 1C, illustrating a typical cross sectional view of a single layer (polymer) OLED. As shown, the device includes a glass substrate 1 coated by a thin ITO layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg, Ca, or other conventionally used low work function metal.
OLEDs can be stacked to form a SOLED, as described in co-pending U.S. Pat. No. 5,707,745, which is incorporated by reference. The SOLED architecture is useful for fabricating low-voltage, color-tunable pixels with independent control of brightness and gray scale, and offers the advantages of minimum pixel size, maximum fill factor and a simple fabrication process. The three-color SOLED illustrates the unique versatility of organic thin film technology to construct highly complex and heterogeneous multilayer systems which are not possible to attain with conventional, inorganic semiconductor technologies. The SOLED pixel architecture can be used in full color flat panel display applications.